[MIPS] Ocelot G: Fix build error and numerous warnings.

The cause of the build errors was a 64-bit kernel being configured in
ocelot_g_defconfig without the code being 64-bit proof.  Fixed for now
by limiting 64-bit selection to SYS_SUPPORTS_64BIT_KERNEL if BROKEN.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>